Wavefront aberration measurements can be used to determine defects in a patient's vision. The correction of vision defects can be expressed by an ophthalmic prescription (Rx), which includes three parameters that correct second order aberrations of a patient's vision. The three parameters can be represented by values for sphere, cylinder, and cylinder axis. Conventional methods, e.g., as described in U.S. Pat. Nos. 7,029,119, 7,077,522, and 7,744,217, calculate the Rx based only on a single fixed pupil size for the eye.